[llvm-dev] [Debuginfo][DWARF][LLD] Remove obsolete debug info in lld.

David Blaikie via llvm-dev
Tue, 09 Jun 2020 00:25:22 -0000

"On Thu, Jun 4, 2020 at 3:11 PM Robinson, Paul <paul.robinson at
sony.com> wrote:
>
> + Ben Dunbobbin, whose name I take in vain below.
> He's my local expert on weird ELF features.
>
> > -----Original Message-----
> > From: David Blaikie <dblaikie at gmail.com>
> > Sent: Thursday, June 4, 2020 2:43 PM
> > To: Robinson, Paul <paul.robinson at sony.com>
> > Cc: jh7370.2008 at my.bristol.ac.uk; llvm-dev at lists.llvm.org
> > Subject: Re: [llvm-dev] [Debuginfo][DWARF][LLD] Remove obsolete debug
info
> > in lld.
> >
> > On Thu, Jun 4, 2020 at 8:27 AM Robinson, Paul <paul.robinson at
sony.com>
> > wrote:
> > >
> > >
> > >
> > > > -----Original Message-----
> > > > From: David Blaikie <dblaikie at gmail.com>
> > > > Sent: Wednesday, June 3, 2020 5:31 PM
> > > > To: Robinson, Paul <paul.robinson at sony.com>
> > > > Cc: jh7370.2008 at my.bristol.ac.uk; llvm-dev at
lists.llvm.org
> > > > Subject: Re: [llvm-dev] [Debuginfo][DWARF][LLD] Remove
obsolete debug
> > info
> > > > in lld.
> > > >
> > > > On Wed, Jun 3, 2020 at 6:34 AM Robinson, Paul
<paul.robinson at sony.com>
> > > > wrote:
> > > > >
> > > > > DWARF was designed in an era when COMDAT and ICF were
not a thing,
> > or at
> > > > least not common, certainly not when talking about function
code.  The
> > > > overhead of a unit occurred only once per translation unit,
so that
> > > > expense was reasonably amortized.
> > > > >
> > > > >
> > > > >
> > > > > Splitting functions into their own object-file sections
and making
> > them
> > > > excludable is an evolution of compiler/linker technology
that DWARF
> > has
> > > > not kept up with.  The linker-friendly solutions (COMDAT
DWARF) would
> > put
> > > > function-related .debug_* contributions into a section-group
along
> > with
> > > > the function .text itself; this multiplies the total number
of
> > sections to
> > > > deal with, regardless of the tactics used for the content of
each per-
> > > > function DWARF section.  The fully DWARF-conformant solution
would
> > create
> > > > one partial_unit per function, with the corresponding
overhead of unit
> > > > headers (especially painful in the .debug_line section).
> > Alternatively we
> > > > fragment DWARF into sections without headers and rely on the
linker to
> > > > make everything look right in the linked executable; this
produces .o
> > > > files that are not DWARF conformant (unless we can
standardize this in
> > > > DWARF v6) and would be a big hassle for consumers other than
the
> > linker.
> > > >
> > > > "object files don't contain DWARF, but they contain
stuff that the
> > > > linker will turn into DWARF" wouldn't seem like the
worst thing to me
> > > > - what sort of pre-linking parsing of DWARF use cases do you
have in
> > > > mind, other than for our own compiler development uses?
> > >
> > > No, that wouldn't seem like the worst thing. Obviously
llvm-dwarfdump
> > > would want to be able to report what's actually happening,
but indeed
> > > all the other use-cases that come to mind are not looking at .o
files.
> > >
> > > > (notwithstanding in-object Split DWARF (where the .dwo
sections would
> > > > have to be remain usable without linking) or the MachO style
debug
> > > > info distribution model which is similar)
> > >
> > > I expect Split DWARF would be incompatible with fragments.  I
don't
> > > know details about MachO  but seems likely the same is true
there.
> >
> > Yep, if they're sub-contribution regions, that wouldn't play
well with
> > Split DWARF. (& full contribution isolation have the DWARF header
> > overhead, etc)
> >
> > I'd still be concerned about the ELF header overhead even of this
> > sub-contribution scheme, but could be interesting to see how it plays
> > out in practice.
> >
> > All that said, to avoid burying the lede here, I'll splice
something
> > from the end up here:
> >
> > > Although the point is not to avoid tombstone values, but to do a
more
> > efficient job of editing the final DWARF to omit gc'd functions;
it's no
> > problem at all to use a tombstone value in .debug_addr IMO.
> >
> > But the tombstone values are Alexey's underlying issue (this
ongoing
> > design discussion for over a year now) & /sort/ of mine too
recently
> > (which, unfortunately, is what's reinvigoraetd this discussion -
> > would've been nice if I/we/someone had identified this sooner
&
> > could've helped Alexey in a more timely manner): Alexey is dealing
> > with a platform where 0 is a valid address so the lld/gold strategy of
> > resolving relocations to dead code to "0+addend" creates
ambiguous
> > DWARF. I'm dealing with a case of zero-length functions ("int
f1() {
> > }" or "void f2() { __builtin_unreachable(); }") causing
early
> > termination of DWARFv4 range lists.
> >
> > The reason for the DWARF-aware linker proposal was because the
"let's
> > choose a better tombstone" discussion didn't go anywhere
& people sort
> > of encouraged in this direction of "what if we didn't need a
> > tombstone/the linker fixed up the debug info instead". So if the
DWARF
> > redundancy elimination doesn't address the issue of zero as a
valid
> > address, it doesn't address Alexey's needs, unfortunately. :/
>
> But, upthread we had a tombstone discussion IIRC, which seemed to converge
> on "-1 except .debug_loc/.debug_ranges use -2" didn't it?  If
we're still
> going on about having the linker rewriting DWARF, then the fragmenting
> idea is worth pursuing as an alternative to Alexey's current work.
>
> >
> > That said, I super appreciate the time you've put into writing
this up
> > and it is valuable & I'd love to see some (even hand-crafted
assembly)
> > prototypes, maybe do some back-of-the-envelope numbers to see whether
> > the ELF header overhead would be worth it, etc.
>
> It would be nice to verify that the section-fragment idea would produce
> something that looked usable.  Hand-written assembly... would require
> research into how to specify the right section attributes, but would
> likely be less effort than trying to make LLVM do something plausible.
>
> I'll see about creating an internal task for this.
>
> >
> > > > But even then, I'm not sure how viable it would be - as
Fangrui
> > > > pointed out on another thread about this: ELF section
overhead itself
> > > > is non-trivial ("sizeof(Elf64_Shdr) = 64.") &
it would probably be
> > > > rather difficult to reconstruct header-less
slice-and-dicable sections
> > > > in some cases. For type information (a reduced overhead
version of
> > > > -fdebug-types-section) I could see it - but for functions,
they need
> > > > to refer to addresses - preferably in the debug_addr
section, and
> > > > that's accessed by index, so taking chunks out of it
would break other
> > > > references to it, etc... adding the header would be
expensive, and how
> > > > would the CU construct its DW_AT_ranges value if that has to
be sliced
> > > > and diced? Again, some amount of linker magic might solve
some of
> > > > these problems - but I think there's still a lot of
overhead to making
> > > > a solution that's workable with a DWARF-agnostic linker
(or even with
> > > > a DWARF aware one, but in an efficient amount of time/space
where it's
> > > > not only usable for small programs, or for linking when
you're
> > > > shipping a final production binary, etc)
> > >
> > > The idea we have blue-skied internally would work something like
this
> > > (initially explicated in terms of the .debug_info section, then
seeing
> > > how that tactic applies to other sections):
> > >
> > > There's a top fragment, containing the CU header and the CU
DIE itself.
> > > Linker magic makes this first in the output file.
> >
> > Quick curiosity: Is there existing linker magic for this? What does it
> > look like? I'd love to know so I can play around with hand crafted
> > prototypes/keep it in mind for such things.
>
> Ben Dunbobbin did research into this some time ago, under the auspices
> of a "COMDAT DWARF" investigation. He's part of Sony's
linker team, and
> it was a discussion with that team where I became convinced that the
> fragmenting idea was feasible using existing defined ELF capabilities,
> although perhaps in ways nobody had really taken advantage of.  It
> involved section groups and/or section ordering, but somebody much more
> familiar with ELF than I am would have to explain it. I've cc'd
Ben.
>
> Regarding my discussion with our linker team:
> They asked me whether it was feasible to use sections to subset the
> DWARF, and I described the functional need (top & bottom fragments,
> arbitrary stuff in between) and they thought the ELF section-group
> and/or section-ordering features would be able to provide that.
>
> I'm not aware that anyone actually tried prototyping that.  The work
> that James did (mentioned upthread) IIRC was using COMDAT and full
> units with unit headers.  My fading memory suggests the discussion
> described just above was after that.

Actually the thread Fangrui linked got me thinking & so I poked
around, according to <
https://docs.oracle.com/cd/E19683-01/816-1386/chapter6-94076/index.html>
 it actually doesn't require as much magic as I thought it might:

"In the absence of the sh_link ordering information, sections from a
single input file combined within one section of the output file will
be contiguous and have the same relative ordering as they did in the
input file. The contributions from multiple input files will appear in
link-line order."

OK, so at the basic level, we could have a debug_info starting
section, some number of comdat debug_info sections, and the debug_info
tail section (the null terminator).

Keeping types alive/cross-referencing them might be trickier (since
their liveness isn't just tied to an existing real comdat), but
do-able.

Yeah, might try and work that up one of these days...

Turns out I decided to do it now. Some of this relates to
threaded-later-than-this-email replies I've read by the time I'm
writing this email.

Yep - for DIEs that don't need to be referenced (such as subprogram
DIEs - assuming they aren't the target of a call_site - all global
variable DIEs (don't think there's any way to target them in LLVM's
current DWARF emission) using comdats and relying on the linker's
guarantee (which is at least documented for some Unix linkers: "In the
absence of the sh_link ordering information, sections from a single
input file combined within one section of the output file will be
contiguous and have the same relative ordering as they did in the
input file. The contributions from multiple input files will appear in
link-line order." -
https://docs.oracle.com/cd/E19683-01/816-1386/chapter6-94076/index.html
"When not otherwise constrained, sections should be emitted in input
order." -
https://web.stanford.edu/~ouster/cgi-bin/cs140-spring18/pintos/specs/sysv-abi-update.html/ch4.sheader.html
)

So that works a treat, exactly as Paul suggested.

Also works to modify the debug_ranges/rnglists section keeping list
fragments in the same comdat group too - though the object size
overhead there is probably higher than worthwhile, given the size of
the contributions versus the size of ELF sections/groups/etc. If
someone really wants to trim their ranges/rnglists down for size - I
think a linker feature would be suitable there, because the section is
small/format is simpler (hmm, except addrx forms - those would require
parsing CU DIEs to get the addr_base to know which addresses were
referenced by the range list, etc... :/).

Doing this with types is a bit more difficult (yes, type units exist,
but I was wondering if we could avoid some of their overhead with
techniques like this) - I'm not sure how to make a hunk of .debug_info
that gets dropped if no symbol in it is referenced. (-gc-sections
didn't seem to activate on the hunk of .debug_info I tried... I guess
if that worked then all the debug_info would be dropped all the time -
so we'd need some way to specifically opt-in ). Attempts at using an
external symbol so type hunks could be deduplicated by comdat, with
cross-CU references resolved by symbol - but that doesn't seem to have
worked out (the final value used for the symbolic reference is not the
desired value... I'm probably holding it wrong).




> > (basically the ability for an object file to say "here's the
start and
> > end of my contribution to this section, and some bits that /can/ go in
> > the middle, but you can drop them if you like")
> >
> > > Types also go here; certainly base types, and other file-scope
types
> > > can be included here or put into type units.  (Type units
aren't
> > > fragmented, they are their own thing same as always.)
> >
> > Separately, it might be worth considering putting types in such a
> > thing - but, yes, the "How do you reference them when they might
be in
> > your unit or someone else's unit", etc, would have to be
figured out.
> > I guess using an external symbol might be the solution there - again,
> > with a better understanding of the ^ mentioned linker magic, I'd
> > probably play around with hand crafting some examples just to see how
> > this could work.
> >
> > > There's a matching bottom fragment, which is just the
terminating NULL
> > > for the CU DIE; linker magic makes this last in the output file.
> >
> > Last of all the contributions from this object file, not last in the
> > whole output file, right? (please excuse the pedantry, just double
> > checking)
>
> The object file would (loosely speaking) have a
".debug_info.first",
> some number of ".debug_info.excludable-middle", and a
".debug_info.last"
> which would all be glommed together in first-middle-last order in the
> output .debug_info section.  I believe I was told that this would be
> per-object-file, otherwise yeah it wouldn't work at all.
>
> This is why we need input from somebody who actually knows ELF.
>
> >
> > > Each function has its own fragment, which is in the same
link-group
> > > (COMDAT or whatever) as the function's .text section; that
way, if the
> > > function is discarded, so is the .debug_info fragment.  Offhand I
can't
> > > think of any cases (other than DW_AT_specification, addressed
below) of
> > > references to a subprogram DIE from elsewhere,
> >
> > The call_site DWARF would want to refer to a subprogram DIE, but that
> > could be handled by (first pass) having a declaration subprogram in
> > the initial fragment that the call_site could refer to using the usual
> > assembler-resolved CU-relative offset. Of course that'd mean a
bunch
> > of (probably the bigger part) of the function's DWARF footprint
> > wouldn't be deduplicated, but would address this part of the
address
> > tombstone issue (if not using debug_addr) & reduce some of the
DWARF -
> > the addresses are pretty big (if you're not pooling them), etc.
>
> Ah, forgot about call_site.  Yeah referring to a declaration should work.
>
> >
> > > so it should be fine to
> > > discard the entire function fragment as needed.  Linker magic
puts all
> > > function fragments between the top and bottom fragments, in some
> > > indeterminate order.  Each function fragment is the usual
complete
> > > subtree, rooted in DW_TAG_subprogram.
> >
> > Rooted at the top level (well, below the DW_TAG_compile_unit) DIE, as
> > you mention later - namespace, or whatever else.
>
> Right, each fragment would be a complete subtree that would ordinarily
> be a direct child of DW_TAG_compile_unit.  With whatever DIE it needed.
>
> >
> > >  References to types are either
> > > to type units as normal, or to types in the top fragment.  Note
that
> > > these references do not require relocations; type units are by
signature
> > > as always, and for types in the top fragment, the offsets into
the top
> > > fragment are known at compile time.
> > >
> > > Inlined functions are described as part of the function they have
been
> > > inlined into, being children of the function DIE.
DW_AT_specification
> > > refers to the abstract declaration which is in its own fragment
(or the
> > > top fragment, but that keeps the declaration from being elided if
all
> > > references go away).
> >
> > Yep, this overlaps with the call_site stuff I mentioned earlier - same
> > ideas. Either top fragment, or its own fragment. Keeping its own
> > fragment alive, and figuring out how to reference it (depending on
> > fragment layout/elision) would require some work, but I think it's
> > do-able. Might even be do-able so it can be deduplicated across CUs
> > (use a sec_offset form, use a linker-resolved relocation to it) - this
> > infrastructure would overlap with type deduplication without type
> > units too.
> >
> > Though linker resolved relocations add more bytes...
> >
> > > If functions are inside namespaces, each function fragment will
need
> > > to have namespace DIEs around the function DIE.  This adds
overhead
> > > but it's pretty small.
> > >
> > > I hand-wave filling in the CU header's unit length.  I'd
expect a
> > > relocation with a reference to the bottom fragment should be able
to
> > > compute the correct value.
> >
> > *nod*
> >
> > > That's the story for .debug_info; what about other sections?
> > >
> > > Sections referenced by index from .debug_info can't be
fragmented;
> > > this would be: .debug_abbrev, .debug_addr, .debug_str_offsets.
> > >
> > > .debug_str doesn't need to be fragmented, linkers DTRT
already.
> >
> > (linkers deduplicate debug_str - but can they be made to remove
> > unreferenced strings too? in that cas ewe'd have an interesting
> > tradeoff of maybe using FORM_strp rather than strx - if we wanted the
> > linker to be able to drop strings from dropped function definitions,
> > etc)
>
> Future refinements are quite possible!
>
> >
> > > .debug_macro contents are not tied to functions and won't be
fragmented.
> > >
> > > .debug_loclists and .debug_rnglists should be fragmentable the
same
> > > way as .debug_info; they exist only as extensions of .debug_info,
and
> > > the range list for the CU itself is merely a concatenated set of
> > > contributions from each constituent function, so that should Just
Work
> > > (although it won't be optimal, adjacent ranges won't be
coalesced).
> >
> > At least the way we currently emit loclists and rnglists is by using
> > an index (the header of loclists and rnglists has an index to offset
> > mapping) - like strx, this would make it hard/impossible for a
> > DWARF-agnostic linker to see through to find out which indexes were
> > actually used. We could potentially not use the loclistx/rnglistx
> > forms/indexes from fragments - instead using sec_offsets that would
> > make them relocatable/removable/etc. (so long as all the index-based
> > referenced lists came in the debug_loclist/debug_rnglist header
> > fragment)
>
> Ah, I hadn't looked at how we do those lists.  But sounds solvable.
>
> >
> > > I believe the same is true for .debug_loc and .debug_ranges,
although
> > > I haven't checked.
> >
> > Yep, those ones are easier - there's no contribution header, they
can
> > only be referenced via sec_offset, so slicing and dicing them is
> > cheap.
> >
> > But the tombstone problem still exists for the CU's debug_ranges -
> > though /maybe/ it could be carefully constructed from fragments...
> > that's going to be a /lot/ of sections in the end though.
> >
> > > .debug_aranges is functionally equivalent to the CU rangelist.
> >
> > Yup. (as we've touched on before, we don't use aranges at
Google -
> > instead relying on CU's ranges which are just a little more
expensive
> > to retrieve - but no need to duplicate the data in both places - if
> > consumers really find the aranges worthwhile to avoid parsing a few
> > attributes on the CU DIE, perhaps a future spec could let
> > debug_aranges reference a range list? so that aranges and the CU could
> > share the same data?)
> >
> > > .debug_line can work the same way as .debug_info but is worth a
word.
> > > The top fragment has the header, including the directory/file
lists
> > > because those are referenced by index.  DW_LNE_define_file
can't be
> > > used.  Each function has a fragment containing the sequence for
that
> > > function, starting with set_address and ending with end_sequence.
> > > The bottom fragment is empty, existing only to allow the length
to
> > > be computed.
> >
> > Yep - can't remove dead file and directory names, unfortunately -
and
> > the line table's pretty compact, so not sure it'd be a great
savings
> > (especially compared to the ELF section overhead - at the object file
> > size at least (though probably a small win for linked executable
> > size)). Chances are those strings (now in debug_line_str) would be
> > used /somewhere/ in the program, so linker string deduplication would
> > get most of the wins - just dead offset entries in the line table
> > header.
>
> Sony does squeeze out the sequences for dead functions; I think it's
> not a huge win, in terms of total debug info size, but the .debug_line
> section does not let you skip dead sequences; you still have to parse
> the whole thing.  Our debugger guys were pleased at not having to
> spend time doing something that useless.  (Yeah it does mean the
> linker has to parse the whole .debug_line section; but our theory is
> that you probably run the debugger more than you run the linker, and
> in any case you do it interactively, so debugger load time is probably
> more annoying than some fractional increase in build/link time.)
>
> The dir/file tables can't be squeezed, but one expects it's not a
> huge cost with .debug_line_str having lots of deduplication
> opportunities.
>
> >
> > > .debug_line_str is a string section and requires nothing special.
> > >
> > > .debug_names ... haven't looked at it but I suspect either it
doesn't
> > > survive or it has to be generated post-link (or by the linker).
> >
> > Generally you're going to want a DWARF-aware linker for
debug_names,
> > same as gdb-index, etc.
> >
> > > .debug_frame I *think* can be fragmented, but I haven't take
the
> > > time to look at it to make sure.
> > >
> > > Those are all the sections I see in DWARF v5 Appendix B.
> > >
> > > So that's the blue-sky vision of linker-magic COMDAT DWARF,
which
> > > took me about an hour to write down just now.  There is certainly
> > > a non-trivial overhead in terms of ELF sections; in the general
> > > case we would have 5 per-function fragments (for .debug_info,
> > > .debug_line, .debug_rnglists, .debug_loclists, .debug_aranges).
> > >
> > > Not small, but then other features in the works are using huge
> > > quantities of ELF sections too (section-per-basic-block).
> >
> > That work's being scoped to be fairly selective about which basic
> > blocks it puts in unique sections - just those that are especially
> > performance sensitive, so the cost isn't as high as you might
> > otherwise imagine. Adding 5 new sections per function would be
> > probably a significantly larger growth than anything else I'm
aware
> > of, but I haven't run the numbers by any means.
>
> Doing it for *every* function would be the worst case, for when
> you're trying to squeeze everything (gc + icf).  We could likely
> get wins if we did it just for the functions that today end up in
> a COMDAT section (inline functions, template instantiations) which
> previous research has found to be pretty significant (and major
> motivation for the Program Repository work that we've previously
> described at a Dev Meeting, https://llvm.org/devmtg/2016-11/#talk22)
>
> >
> > Thanks again for the write up!
>
> NP, it was fun to trot out this stuff.
> --paulr
>
> >
> > - Dave
> >
> > > > & as always, not sure how any of this would work for
Split DWARF -
> > > > just a debug_adr section that has some addresses that point
to
> > > > discardable functions... if we want those addresses
themselves to be
> > > > discardable (so we don't have to use a tombstone value
inserted by the
> > > > linker) then they'd need to be in separate debug_addr
contributions
> > > > with headers, etc - the overhead just seems too high to me
in all the
> > > > ways I can look at that.
> > >
> > > Yeah I think .dwo sections can't take advantage of
fragmenting, and
> > > .debug_addr is referenced by index so it can't be fragmented.
Although
> > > the point is not to avoid tombstone values, but to do a more
efficient
> > > job of editing the final DWARF to omit gc'd functions;
it's no problem
> > > at all to use a tombstone value in .debug_addr IMO.
> > > --paulr
> > >
> > > >
> > > > >
> > > > >
> > > > >
> > > > > Or we pay the cost of parsing, trimming, and rewriting
all the DWARF
> > in
> > > > the linker.
> > > > >
> > > > > --paulr
> > > > >
> > > > >
> > > > >
> > > > > From: llvm-dev <llvm-dev-bounces at
lists.llvm.org> On Behalf Of James
> > > > Henderson via llvm-dev
> > > > > Sent: Wednesday, June 3, 2020 3:48 AM
> > > > > To: David Blaikie <dblaikie at gmail.com>
> > > > > Cc: llvm-dev at lists.llvm.org
> > > > > Subject: Re: [llvm-dev] [Debuginfo][DWARF][LLD] Remove
obsolete
> > debug
> > > > info in lld.
> > > > >
> > > > >
> > > > >
> > > > > It makes me sad that the linker (via a library or
otherwise) has to
> > be
> > > > "DWARF-aware" to be able to effectively handle
--gc-sections, COMDATs,
> > --
> > > > icf etc for debug info, without leaving large blocks of data
kicking
> > > > around.
> > > > >
> > > > >
> > > > >
> > > > > The patching to -1 (or equivalent) is probably a good
lightweight
> > > > solution (though I'd love it if it could be done based
on section type
> > in
> > > > the future rather than section name, but that's probably
outside the
> > realm
> > > > of DWARF), as it requires only minimal understanding in the
linker,
> > but
> > > > anything beyond that seems to be complicated logic that is
mostly due
> > to
> > > > the structure of DWARF. Patching to -1 does feel a bit like
a sticking
> > > > plaster/band aid to patch over the issue rather than
properly solving
> > it
> > > > too - there will still be debug data (potentially
significant amounts
> > in
> > > > COMDAT-heavy objects) that the linker has to write and the
debugger
> > has to
> > > > somehow know how to skip (even if it knows that -1 is
special-case due
> > to
> > > > the standard being updated, it needs to get as far as the
-1), which
> > is
> > > > all wasted effort.
> > > > >
> > > > >
> > > > >
> > > > > We've already seen from Alexey's prototyping,
and from our own
> > > > experiences with the Sony proprietary linker (which tried to
rewrite
> > > > .debug_line only) that deconstructing the DWARF so that it
can be more
> > > > optimally reassembled at link time is slow going, and will
probably
> > > > inevitably be however much effort is put into optimising it.
For a
> > start,
> > > > given the current standards, it's impossible to know how
to
> > deconstruct it
> > > > without having to parse vast amounts of DWARF, which is
typically
> > going to
> > > > mean a lot more parsing work than the linker would normally
have to
> > deal
> > > > with. Additionally, much of this parsing work is wasted
effort, since
> > it
> > > > seems unlikely in many links that large amounts of the DWARF
will be
> > > > redundant. Having an option to opt-in doesn't help much
there, since
> > it
> > > > just means the logic exists without most people using it,
due to it
> > not
> > > > being good enough, or potentially they don't even know
it exists.
> > > > >
> > > > >
> > > > >
> > > > > I don't have particularly concrete suggestions as
to how to solve
> > the
> > > > structural problems with DWARF at this point. The only thing
that
> > seems
> > > > obvious to me is a more "blessed" approach to
fragmentation of
> > sections,
> > > > similar to what I tried with my prototype mentioned earlier
in the
> > thread,
> > > > although we'd need to figure out the previously stated
performance
> > issues.
> > > > Other ideas might tie into this, like somehow sharing the
various
> > table
> > > > headers a bit like CIEs in .eh_frame that could be merged by
the
> > linker -
> > > > each object could have separate table header sections, which
are
> > > > referenced by the individual .debug_* blocks, which in turn
are one
> > per
> > > > function/data piece and easily discardable/merged by the
linker.
> > > > >
> > > > >
> > > > >
> > > > > Just some thoughts.
> > > > >
> > > > >
> > > > >
> > > > > James
> > > > >
> > > > >
> > > > >
> > > > > On Tue, 2 Jun 2020 at 19:24, David Blaikie via llvm-dev
<llvm-
> > > > dev at lists.llvm.org> wrote:
> > > > >
> > > > > On Tue, May 19, 2020 at 7:17 AM Alexey Lapshin
> > > > > <alapshin at accesssoftek.com> wrote:
> > > > > >
> > > > > > Hi David, please find my comments inside:
> > > > > >
> > > > > >
> > > > > > >>>Broad question: Do you have any
specific motivation/users/etc
> > in
> > > > implementing this (if you can speak about it)?
> > > > > >
> > > > > > >>> - it might help motivate the work,
understand what tradeoffs
> > might
> > > > be suitable for you/your users, etc.
> > > > > >
> > > > > > >>There are two general requirements:
> > > > > > >> 1) Remove (or clean) invalid debug info.
> > > > > >
> > > > > > >
> > > > > > >Perhaps a simpler direct solution for your
immediate needs might
> > be a
> > > > much narrower,
> > > > > > >and more efficient linker-DWARF-awareness
feature:
> > > > > > >
> > > > > > > With DWARFv5, rnglists present an opportunity
for a DWARF linker
> > to
> > > > rewrite the ranges
> > > > > > > without parsing the rest of the DWARF.
/technically/ this isn't
> > > > guaranteed - rnglist entries
> > > > > > > can be referenced either directly, or by
index. If all rnglists
> > are
> > > > referenced by index, then
> > > > > > > a linker could parse only the debug_rnglists
section and rewrite
> > > > ranges to remove any
> > > > > > > address ranges that refer to optimized-out
code.
> > > > > > >
> > > > > > > This would only be correct for rnglists that
had no direct
> > > > references to them (that only were
> > > > > > > referenced via the indexes) - but we could
either implement it
> > with
> > > > that assumption, or could
> > > > > > > add an LLVM extension attribute on the CU
that would say "I
> > promise
> > > > I only referenced rnglists
> > > > > > > via rnglistx forms/indexes). If this
DWARF-aware linking would
> > have
> > > > to read the CU DIE (not
> > > > > > > all the other DIEs) it /could/ also then
rewrite high/low_pc if
> > the
> > > > CU wasn't using ranges...
> > > > > > > but that wouldn't come up in the
function-removal case, because
> > then
> > > > you'd have ranges anyway,
> > > > > > > so no need for that.
> > > > > > >
> > > > > > > Such a DWARF-aware rnglist linking could also
simplify rnglists,
> > in
> > > > cases where functions
> > > > > > > ended up being laid out next to each other,
the linker could
> > > > coalesce their ranges together.
> > > > > > >
> > > > > > > I imagine this could be implemented with very
little overhead to
> > > > linking, especially compared
> > > > > > > to the overhead of full DWARF-aware linking.
> > > > > > >
> > > > > > >Though none of this fixes Split DWARF, where
the linker doesn't
> > get a
> > > > chance to see the
> > > > > > > addresses being used - but if you only
want/need the CU-level
> > ranges
> > > > to be correct, this
> > > > > > > might be a viable fix, and quite efficient.
> > > > > >
> > > > > > Yes, we think about that alternative. This would
resolve our
> > problem
> > > > of invalid debug info
> > > > > > and would work much faster. Thus, if we would not
have good
> > results
> > > > for D74169 then we
> > > > > > will implement it. Do you think it could be useful
to have this
> > > > solution in upstream?
> > > > >
> > > > > A pure rnglist rewriting - I think it'd be OK to
have in upstream -
> > > > > again, cost/benefit/etc would have to be weighed.
I'm not sure it
> > > > > would save enough space to be particularly valuable
beyond the
> > > > > correctness issue - and it doesn't completely solve
the correctness
> > > > > issue for zero-address usage or low-address usage
(because you could
> > > > > still have overlapping subprograms inside a CU - so if
you were
> > > > > symbolizing you could use the correct rnglist to
filter, but then go
> > > > > look inside the CU only to find two subprograms that
had that
> > address
> > > > > & not know which one was the correct one an which
one was the
> > > > > discarded one).
> > > > >
> > > > > rnglist rewriting might be easy enough to prototype -
but depends
> > what
> > > > > you want to spend your time on, I know this whole issue
has been a
> > > > > huge investment of your time already - but maybe this
recent
> > > > > revitalization of the conversation around having an
explicit value
> > in
> > > > > the linker might be sufficient to address
everyone's needs...
> > *fingers
> > > > > crossed*)
> > > > >
> > > > >
> > > > > > >> 2) Optimize the DWARF size.
> > > > > >
> > > > > >
> > > > > > > Do your users care much about this? I imagine
if they had
> > > > significant DWARF size issues,
> > > > > > > they'd have significant link time issues
and the kind of cost to
> > > > link time this feature has would
> > > > > > > be prohibitive - but perhaps they're
sharing linked binaries
> > much
> > > > more often than they're
> > > > > > > actually performing linking.
> > > > > >
> > > > > > Yes, they do. They also have significant link-time
issues.
> > > > > > So current performance results of D74169 are not
very acceptable.
> > > > > > We hope to improve it.
> > > > > >
> > > > > >
> > > > > >
> > > > > > >>The specifics which our users have:
> > > > > > >>  - embedded platform which uses 0 as
start of .text section.
> > > > > > >>  - custom toolset which does not support
all features yet(f.e.
> > > > split dwarf).
> > > > > > >>  - tolerant of the link-time increase.
> > > > > > >>  - need a useful way to share debug
builds.
> > > > > >
> > > > > >
> > > > > > > Sharing two files (executable and dwp) is
significantly less
> > useful
> > > > than sharing one file?
> > > > > >
> > > > > > Probably not significantly, but yes, it looks less
useful
> > comparing to
> > > > D74169.
> > > > > > Having only two files (executable and .dwp) looks
significantly
> > better
> > > > than having executable and multiple .dwo files.
> > > > > > Having only one file(executable) with minimal size
looks better
> > than
> > > > the two files with a bigger size.
> > > > > >
> > > > > > clang compiled with -gsplitdwarf takes 0.9G for
executable and
> > 0.9G
> > > > for .dwp.
> > > > > > clang compiled with -gc-debuginfo takes only 0.76G
for single
> > > > executable.
> > > > > >
> > > > > >
> > > > > >
> > > > > > >>For the first point: we have a problem
"Overlapping address
> > ranges
> > > > starting from 0"(D59553).
> > > > > >
> > > > > > >>We use custom solution, but the general
solution like D74169
> > would
> > > > be better here.
> > > > > >
> > > > > >
> > > > > > > If CU ranges are the only ones that need
fixing, then I think
> > the
> > > > above solution might be as
> > > > > > > good/better - if more than CU ranges need
fixing, then I think
> > we
> > > > might want to start talking about
> > > > > > > how to fix DWARF itself (split and non-split)
to signal certain
> > > > addresses point to dead code with a
> > > > > > > specific blessed value that linkers would
need to implement -
> > > > because with Split DWARF there's
> > > > > > > no way to solve the non-CU addresses at the
linker.
> > > > > >
> > > > > > I think the worthful solution for that signal
value would be LowPC
> > >
> > > > HighPC.
> > > > > > That does not require additional bits in DWARF.
> > > > > > It would be natural to skip such address ranges
since they
> > explicitly
> > > > marked as invalid.
> > > > > > It could be implemented in a linker very easily.
Probably, it
> > would
> > > > make sense to describe that
> > > > > > usage in DWARF standard.
> > > > > >
> > > > > > As to the addresses which are not seen by the
linker(since they
> > are in
> > > > .dwo files) - yes,
> > > > > > they need to have another solution. Could you show
an example of
> > such
> > > > a case, please?
> > > > > >
> > > > > >
> > > > > >
> > > > > > >>>2. Support of type units.
> > > > > >
> > > > > > >>>
> > > > > >
> > > > > > >>>>  That could be implemented
further.
> > > > > >
> > > > > > >>>Enabling type units increases object
size to make it easier to
> > > > deduplicate at link time by a DWARF-unaware
> > > > > >
> > > > > > >>>linker. With a DWARF aware linker
it'd be generally desirable
> > not
> > > > to have to add that object size overhead to
> > > > > >
> > > > > > >>>get the linking improvements.
> > > > > >
> > > > > > >>
> > > > > >
> > > > > > >>But, DWARFLinker should adequately work
with type units since
> > they
> > > > are already implemented.
> > > > > >
> > > > > >
> > > > > > > Maybe - it'd be nice & all, but I
don't think it's an outright
> > > > necessity - if someone knows they're using
> > > > > > > a DWARF-aware linker, they'd probably not
use type units in
> > their
> > > > object files. It's possible someone
> > > > > > > doesn't know for sure & maybe they
have pre-canned debug object
> > > > files from someone else, etc.
> > > > > >
> > > > > > I see.
> > > > > >
> > > > > > >>Another thing is that the idea behind type
units has the
> > potential
> > > > to help Dwarf-aware linker to work faster.
> > > > > >
> > > > > > >>Currently, DWARFLinker analyzes context to
understand whether
> > types
> > > > are the same or not.
> > > > > >
> > > > > >
> > > > > > >When you say "analyzes context" what
do you mean? Usually I'd
> > take
> > > > that to mean
> > > > > > > "looks at things outside the type itself
- like what namespace
> > it's
> > > > in, etc" - which, yes,
> > > > > > > it should do that, but it doesn't seem
very expensive to do. But
> > I
> > > > guess you actually
> > > > > > > mean something about doing structural
equivalence in some way,
> > > > looking at things inside the type?
> > > > > >
> > > > > > I think it could be useful for both cases.
Currently, dsymutil
> > does
> > > > only first thing
> > > > > > (look at type name, namespace name, etc..) and
does not do the
> > second
> > > > thing
> > > > > > (doing structural equivalence). Analyzing type
names is currently
> > > > quite expensive
> > > > > > (the only search in string pool takes ~10 sec from
70 sec of
> > overall
> > > > time).
> > > > > > That is expensive because of many things should be
done to work
> > with
> > > > strings:
> > > > > > parse DWARF, search and resolve relocations,
compute a hash for
> > > > strings,
> > > > > > put data into a string pool, create a fully
qualified name(like
> > > > namespace::function::name).
> > > > > > It looks like it could be optimized and finally
require less time,
> > but
> > > > it still would be a noticeable
> > > > > > part of the overall time.
> > > > > >
> > > > > > If dsymutil starts to check for the structural
equivalence, then
> > the
> > > > process would be even more slowly.
> > > > > > So, If instead of comparing types structure, there
would be
> > checked
> > > > single hash-id - then this process
> > > > > > would also be faster.
> > > > > >
> > > > > > Thus I think using hash-id to compare types would
allow to make
> > > > current implementation faster and would
> > > > > > allow handling incomplete types by DWARFLinker
without massive
> > > > performance degradation also.
> > > > > >
> > > > > > >> But the context is known when types are
generated. So, no need
> > to
> > > > spent the time analyzing it.
> > > > > >
> > > > > > >> If types could be compared without
analyzing context, then
> > Dwarf-
> > > > aware linker would work faster.
> > > > > >
> > > > > > >> That is just an idea(not for immediate
implementation): If
> > types
> > > > would be stored in some "type table"
> > > > > >
> > > > > > >> (instead of COMDAT section group) and
could be accessed through
> > > > hash-id(like type units
> > > > > >
> > > > > > >> - then it would be the solution requiring
fewer bits to store
> > but
> > > > allowing to compare types
> > > > > >
> > > > > > >> by hash-id(not analysing context).
> > > > > > >> In this case, size increasing would be
small. And processing
> > time
> > > > could be done faster.
> > > > > > >>
> > > > > > >> this is just an idea and could be
discussed separately from the
> > > > problem of integrating of D74169.
> > > > > >
> > > > > > >> >> 6. -flto=thin
> > > > > >
> > > > > > >> >>    That problem was described in
this review
> > > >
> >
https://urldefense.com/v3/__https://reviews.llvm.org/D54747*1503720__;Iw!!
> > > >
> >
JmoZiZGBv3RvKRSx!q8U1OiuTHDnORPTzJINrJOLwncHMDEAyE45t99RrMdkDdSYLjh78mgJen
> > > > L-N0pxHMQ$ . It also exists in
> > > > > >
> > > > > > >> >> current DWARFLinker/dsymutil
implementation. I think that
> > > > problem should be discussed more: it could
> > > > > >
> > > > > > >> >> probably be fixed by avoiding
generation of such incomplete
> > > > declaration during thinlto,
> > > > > >
> > > > > > >> >> That would be costly to produce
extra/redundant debug info
> > in
> > > > ThinLTO - actually ThinLTO could be doing
> > > > > >
> > > > > > >> >> more to reduce that redundancy
early on (actually removing
> > > > definitions from some llvm Modules if the type
> > > > > >
> > > > > > >> >> definition is known to exist in
another Module, etc)
> > > > > > >> >I don't know if it's a
problem since that patch was reverted.
> > > > > >
> > > > > > >>
> > > > > >
> > > > > > >> Yes. That patch was reverted, but this
patch(D74169) has the
> > same
> > > > problem.
> > > > > >
> > > > > > >> if D74169 would be applied and
--gc-debuginfo used then
> > structure
> > > > type
> > > > > > >> definition would be removed.
> > > > > >
> > > > > > >> DWARFLinker could handle that case -
"removing definitions from
> > > > some llvm Modules if the type
> > > > > > >> definition is known to exist in another
Module".
> > > > > > >> i.e. DWARFLinker could replace the
declaration with the
> > definition.
> > > > > >
> > > > > > >> But that problem could be more easily
resolved when debug info
> > is
> > > > generated(probably without
> > > > > > >> significant increase of debug info size):
> > > > > >
> > > > > > >> Here we have:
> > > > > >
> > > > > > >> DW_TAG_compile_unit(0x0000000b) - compile
unit containing
> > concrete
> > > > instance for function "f".
> > > > > > >> DW_TAG_compile_unit(0x00000073) - compile
unit containing
> > abstract
> > > > instance root for function "f".
> > > > > > >> DW_TAG_compile_unit(0x000000c1) - compile
unit containing
> > function
> > > > "f" definition.
> > > > > >
> > > > > > >> Code for function "f" was
deleted. gc-debuginfo deletes compile
> > > > unit DW_TAG_compile_unit(0x000000c1)
> > > > > > >> containing "f" definition
(since there is no corresponding
> > code).
> > > > But it has structure "Foo" definition
> > > > > > >> DW_TAG_structure_type(0x0000011e)
referenced from
> > > > DW_TAG_compile_unit(0x00000073)
> > > > > > >> by declaration
DW_TAG_structure_type(0x000000ae). That
> > declaration
> > > > is exactly the case when definition
> > > > > > >> was removed by thinlto and replaced with
declaration.
> > > > > >
> > > > > > >> Would it cost too much if type definition
would not be replaced
> > > > with declaration for "abstract instance root"?
> > > > > > >> The number of concrete instances is
bigger than number of
> > abstract
> > > > instance roots.
> > > > > > >> Probably, it would not be too costly to
leave definition in
> > > > abstract instance root?
> > > > > >
> > > > > >
> > > > > >
> > > > > > >> Alternatively, Would it cost too much if
type definition would
> > not
> > > > be replaced with declaration when
> > > > > > >> declaration references type from not used
function? (lto could
> > > > understand that concrete function is not used).
> > > > > >
> > > > > >
> > > > > > >I don't follow this example - could you
provide a small concrete
> > test
> > > > case I could reproduce?
> > > > > >
> > > > > > I would provide a test case if necessary. But it
looks like this
> > issue
> > > > is finally clear, and you already commented on that.
> > > > > >
> > > > > > > Oh, I guess this is happening perhaps because
ThinLTO can't know
> > for
> > > > sure that a standalone
> > > > > > > definition of 'f' won't be needed
- so it produces one in case
> > one
> > > > of the inlining opportunities
> > > > > > > doesn't end up inlining. Then it turns
out all calls got
> > inlined, so
> > > > the external definition wasn't needed.
> > > > > >
> > > > > > > Oh, you're suggesting that these 3 CUs
got emitted into one
> > object
> > > > file during LTO, but that DWARFLinker
> > > > > > > drops a CU without any code in it - even
though... So far as I
> > know,
> > > > in LTO, LLVM directly references
> > > > > > > types across units if the CUs are all emitted
in the same object
> > > > file. (and if they weren't in the same
> > > > > > > object file - then the abstract_origin
couldn't be pointing
> > cross-
> > > > CU).
> > > > > >
> > > > > > > I guess some basic things to say:
> > > > > >
> > > > > > > With ThinLTO, the concrete/standalone
function definition is
> > emitted
> > > > in case some call sites don't end up
> > > > > > > being inlined. So we know it'll be
emitted (but might not be
> > needed
> > > > by the actual linker)
> > > > > > > ANy number of inline calls might exist - but
we shouldn't put
> > the
> > > > type information into those, because
> > > > > > > they aren't guaranteed to emit it (if the
inline function gets
> > > > optimized away, there would be nothing to
> > > > > > > enforce the type being emitted) - and even if
we forced the type
> > > > information to be emitted into one
> > > > > > > object file that has an inline copy of the
function - there's no
> > > > guarantee that object file will get linked in either.
> > > > > >
> > > > > > > So, no, I don't think there's much we
can do to keep the size of
> > > > object files down, while guaranteeing
> > > > > > > the type information will be emitted with the
usual linker
> > > > semantics.
> > > > > >
> > > > > > Then dsymutil/DWARFLinker could be changed to
handle that(though
> > it
> > > > would probably be not very efficient).
> > > > > > If thinlto would understand that function is not
used finally(and
> > then
> > > > must not contain referenced type definition),
> > > > > > then this situation could be handled more
effectively.
> > > > > >
> > > > > > Thank you, Alexey.
> > > > > >
> > > > > >>>
> > > > > >>>
> > > > > >>>
> > > > > >>>
> > > > > >>>
_______________________________________________
> > > > > >>> LLVM Developers mailing list
> > > > > >>> llvm-dev at lists.llvm.org
> > > > > >>>
https://urldefense.com/v3/__https://lists.llvm.org/cgi-
> > > > bin/mailman/listinfo/llvm-
> > > >
> >
dev__;!!JmoZiZGBv3RvKRSx!q8U1OiuTHDnORPTzJINrJOLwncHMDEAyE45t99RrMdkDdSYLj
> > > > h78mgJenL-Oh8zYPg$
> > > > > _______________________________________________
> > > > > LLVM Developers mailing list
> > > > > llvm-dev at lists.llvm.org
> > > > > https://urldefense.com/v3/__https://lists.llvm.org/cgi-
> > > > bin/mailman/listinfo/llvm-
> > > >
> >
dev__;!!JmoZiZGBv3RvKRSx!q8U1OiuTHDnORPTzJINrJOLwncHMDEAyE45t99RrMdkDdSYLj
> > > > h78mgJenL-Oh8zYPg$